The TNFa/NF-KB signaling pathway is a major regulator of the innate immune response part of which is mediated through its effects upon hematopoiesis. Studies with human and mouse hematopoietic cells support a model in which TNFa inhibits hematopoietic stem and progenitor cell self-renewal by inducing differentiation. In an acute setting the host benefits as TNFa induction of differentiation generates a population of cells (macrophages, neutrophils) who function to clear pathogens and remodel damaged tissue. However under chronic conditions, TNFa can deplete stem and progenitor cell numbers, a physiological state similar to what is seen in myelodysplastic syndromes, which are poised to evolve into leukemia. The fellow's laboratory has identified a novel signaling pathway downstream of the type I TNFa receptor (TNF Rl) required for full NF-KB activity [TNFa?TNF RI?mPLK/IRAK-1?SIMPL?NF-KB]. TNFa activation of the mouse pelle-like kinase [mPLK;also known as the interleukin-1 receptor associated kinase (IRAK 1)] leads to phosphorylation and nuclear relocalization of SIMPL. Nuclear SIMPL synergistically enhances NF-kB transactivation activity. c-kit+ low density bone marrow cells infected with retrovirus expressing a SIMPL shRNA revealed that SIMPL is required for TNFa regulated hematopoietic cell survival. Mice with a targeted disruption of a SIMPL allele (SIMPL-/+) have been generated and provide an ideal opportunity to simultaneously evaluate the biological significance of SIMPL in vivo and to provide a vital training opportunity for Dr. Harrington. The NIH has placed a high priority on research employing model systems focused on stem cell disorders. The applicant has the tools to pursue a biologically significant question, the evolution of myelodysplastic syndromes by addressing the hematopoietic stem cell compartment but needs training in mouse models of experimental hematopoiesis. In the laboratory of the mentor, Dr. Hal Broxmeyer, an internationally recognized leader in experimental hematopoiesis, Dr. Harrington will receive the necessary training to test the hypothesis that loss of SIMPL leads to hematopoietic stem cell depletion, which will be tested by characterizing the hematopoietic system of the SIMPL-/- mice as they age;and assessing the competency of hematopoietic stem cells isolated from the SIMPL-/- mice by measuring their ability to reconstitute myeloablated mice. These studies will provide insight into the mechanism through which TNFa inhibits hematopoietic stem and progenitor cell expansion. Identifying a role for SIMPL in the maintenance of hematopoietic stem cells will equip the applicant with the experimental skills to direct independent research addressing experimental hematopoiesis questions using whole animals.